Discussion
Bandwidth of frequency modulation (FM) is given by :
Where fm = frequency of modulating Signal
fd = deviating frequency
Answer: A
The bandwidth is \(2(f_{m} + f_{d})\), Carson's rule.
An FM signal has, in theory, infinitely many sidebands, but almost all of its power lies within a band of \(2(\Delta f + f_{m})\), where \(\Delta f = f_{d}\) is the frequency deviation and \(f_{m}\) the highest modulating frequency. This approximation is Carson's rule.
For example, broadcast FM with \(f_{d} = 75\) kHz and \(f_{m} = 15\) kHz needs about \(2(75 + 15) = 180\) kHz. \(f_{m} + f_{d}\) is only half of that band, the average \(\frac{f_{m} + f_{d}}{2}\) has no physical meaning here, and \(2f_{m}\) is the bandwidth of AM or of very narrowband FM.
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